The invention relates to a circuit for eliminating noises contained in a signal, for example, to a noise elimination circuit which removes chattering noises which are included in an output signal from a switch having a mechanical contact or sensor.
Various forms of noise elimination circuits are disclosed in Chapter 14 "Elimination of Noises and Detection of Signal" of a book entitled "Dictionary of Patented Pulse Circuit Technology" edited by Suzuki and Higuchi and published from OHM-Sha.
A most fundamental form utilizes C-R circuit, which suffers from a large variation in the error. Recently, an arrangement is utilized in a microcomputer or the like which removes chattering noises by a software processing. However, this involves a disadvantage that the microcomputer cannot be used to perform other operations because it is busy with the elimination of noises. This is inconvenient particularly when a high speed processing operation is desired.
When removing noises from a signal derived from a sensing device as used in a microcomputer or the like, a read timing is established by means of a clock pulse. Hence it is desirable to perform the elimination of noises in synchronism with such clock pulse. One form of an arrangement which eliminates noises in synchronism with a clock pulse is disclosed in Japanese Patent Publication No. 36,571/1987, the claim of which reads as follows:
"An apparatus for detecting a signal depending on a rotational speed used to control a rotational speed and for shaping it into and delivering it as a pulse signal; comprising
a clock input terminal which receives a rotation detection signal; a clock input terminal which receives a reference clock signal; coincidence detection means having its one input terminal connected to the input terminal which receives the rotation detection signal; an R-S flipflop having its set input terminal connected to an output terminal of the coincidence detection means; a counter circuit connected to the clock input terminal and to an output terminal of the R-S flipflop for counting a predetermined number of reference clock signals when the R-S flipflop is set to deliver a trigger pulse to a reset terminal of the R-S flipflop; a flipflop circuit having its input terminal connected to an output terminal of the counter and having its output terminal connected to the other input terminal of the coincidence detection means for inverting the state of an output level each time the trigger pulse is input thereto; and a rotation detection signal output terminal connected to an output terminal of the flipflop circuit to deliver a signal from which noises are eliminated."
In this arrangement, a rotation detection signal which is used to perform a control over the rotational speed is chosen to be a sinusoidal wave, and it is assumed that noises occur around a zero crossover point. The signal is shaped into a pulse form while it includes noises. Several noise pulses are present adjacent to the rising and falling edges of the shaped signal. Of these, the first pulse is detected, and is utilized to derive a rotation detection pulse from which noise pulses are removed.
However in the noise elimination circuit described, if noises occur in the vicinity of a peak of the rotation detection signal which is assumed to be a sinusoidal wave, the flipflop responds thereto to invert its output, resulting in a failure to remove the influence of noises.
Therefore, it is an object of the invention to provide a noise elimination circuit which eliminates noises accurately by a digital processing of a signal without requiring an increased demand upon the software.